The invention relates to a method of preparing a broadband reflective polarizer comprising two or more layers of a polymerized mesogenic material with helically twisted molecular structure and planar orientation. The invention further relates to a broadband reflective polarizer obtainable by such a method. The invention also relates to the use of such a broadband reflective polarizer in liquid crystal displays, projection systems, decorative and security applications.
Reflective films comprising cholesteric liquid crystal materials have been proposed in prior art for a variety of uses, for example as broadband or notch polarizers, as color filters in displays or projection systems, and for decorative purposes, like the preparation of colored image films or cholesteric pigment flakes.
These films usually comprise one or more layers of a cholesteric liquid crystalline material with planar alignment and show selective reflection of light.
The term xe2x80x9cplanar alignmentxe2x80x9d as used in this application in connection with a layer of a liquid crystal material with helically twisted molecular structure means that the material exhibits an orientation wherein the axis of the molecular helix is oriented substantially perpendicular to the plane of the layer, i.e., substantially parallel to the layer normal. This definition also includes orientations where the helix axis is tilted at an angle xe2x89xa610xc2x0, in particular xe2x89xa65xc2x0, very preferably xe2x89xa62xc2x0 relative to the layer normal.
The bandwidth xcex94xcex of the waveband reflected by a reflective film as described above is depending on the birefringence of the mesogenic material xcex94n and the pitch of the molecular helix p according to the equation xcex94xcex=xcex94nxc3x97p. Thus, the bandwidth among other factors is determined by the birefringence of the material.
For an application as broadband polarizer in liquid crystal displays, it is desirable that the bandwidth of the reflective film should comprise a substantial portion of the visible wavelength range, whereas for an application as notch polarizer or as colored reflective film, e.g. in decorative or security applications, often films having a specific reflection color are desired.
In particular broadband reflective polarizers, also known as circular polarizers, which are transmitting circularly polarized light of a broad wavelength band covering a large part of the visible spectrum, are suitable as polarizers for backlit liquid crystal displays.
If unpolarized light is incident on such a reflective polarizer, 50% of the light intensity is reflected as circularly polarized light with the same twist sense as that of the molecular helix, whereas the other 50% is transmitted. The reflected light is depolarized (or its sense of polarization is reversed) in the backlight of the display, and is redirected onto the polarizer. In this manner theoretically 100% of a given waveband of the unpolarized light incident on the reflective polarizer can be converted into circularly polarized light.
The circularly polarized light can be converted into linear polarized light by means of a quarter wave optical retarder and optionally also a compensation film.
Recently reflective polarizers have been developed that comprise a single layer of liquid crystalline material with helically twisted structure and planar orientation wherein the pitch of the molecular helix is varying in a direction normal to the layer, leading to a large bandwidth of the reflected wavelength band. Such polarizers are described for example in EP 0 606 940-A2 and in WO 97/35219.
A simpler way to provide a broadband reflective polarizer is to stack several reflective layers with different reflection wavebands on top of each other to obtain a multilayer polarizer.
Multilayer reflective polarizers have been described in prior art. For example, EP 0 634 674 suggests to prepare a multilayer cholesteric liquid crystal polymer film by bringing together a pair of chiral nematic liquid crystal polymer films, applying pressure, and heating the polymers above their glass transition temperature to allow the films to adhere.
Maurer et al., SID 90 Digest, Vol. 21, pp. 110 (1990) describe a polarizing color filter obtained by combining several polarizing films with different reflection wavelength. For the preparation of each film, a layer of a cholesteric liquid crystal side chain polysiloxane comprising chiral and achiral sidegroups is brought between two glass plates and oriented by shearing at high temperatures.
JP 01-133003-A discloses a polarizing plate that is obtained by lamination of one or more cholesteric liquid crystal polymer layers onto a quarter wave plate. JP 08-271731-A discloses a similar polarizing plate, but wherein the quarter wave plate comprises at least two retardation films having different retardation.
However, the methods of preparing a multilayer cholesteric polarizer as described in the above documents bear several disadvantages. Thus, it is often very difficult, and requires high temperatures, to achieve uniform alignment in a liquid crystal polymer layer. For example, Maurer et al. mention an aligning temperature of 150xc2x0 C., whereas JP 01-133003-A and JP 08-271731-A mention that temperatures well above the glass temperature of the liquid crystal polymers are required. This is especially disadvantageous when polymers with high glass temperatures, like acrylates, styrenes or methacrylates are used, and is highly unsuitable in particular for mass production.
Furthermore, according to the method of multilayer preparation as described e.g. in JP 01-133003-A, the materials have to be selected such that the different layers exhibit different glass temperatures. Thus, when laminating and aligning e.g. a second layer on top of a first layer, the aligning temperature (and thus the glass temperature) of the second layer has to be lower than the glass temperature of the first layer, so as not to affect the uniform orientation of the first layer. This limits severely the choice of suitable materials and makes the production process more complicated.
Therefore, there was a need for a method to prepare a broadband reflective polarizer in an efficient and cost-effective manner that does not have the above mentioned drawbacks, allows better and more easy control of the reflection wavelength and is particularly suitable for mass production.
One aim of the present invention is to provide a method of preparing a broadband reflective polarizer that fulfills the above requirements. Other aims of the invention are immediately evident to a person skilled in the art from the following description.
It has been found that, by providing a method of preparing a broadband reflective polarizer according to the present invention, it is possible to overcome the drawbacks of the methods described in prior art.
Accordingly, the present invention provides a method of preparing a broadband reflective polarizer including two or more layers of a polymerized mesogenic material with helically twisted molecular structure and planar orientation. Each of the layers is prepared by blending a first mixture A with a second mixture B to obtain a chiral polymerizable mesogenic material. The first mixture A may include at least one achiral polymerizable mesogenic compound, optionally a polymerization initiator component, and optionally an organic solvent component. The second mixture B may include at least one chiral polymerizable mesogenic compound, optionally a polymerization initiator component, and optionally an organic solvent component. The method may also include coating a layer of the blended mixtures A and B onto a first substrate or between a first and a second substrate, and aligning the chiral polymerizable mesogenic material in a planar orientation so that the axis of the molecular helix extends transversely to the layer. What is more, the method can include polymerizing the aligned material, and optionally removing the first and, if present, the second substrate from the polymerized layer. Desirably, the ratio of the blended mixtures A and B in each of the layers is chosen to be different from the other layers.